ECC is more about keeping a memory error from causing the system to crash, which is always bad, or the possibility that good data is corrupted in memory before being recorded to disk. If the data is corrupted in flight, before ZFS can checksum it, then you could have bad data that ZFS is not aware of and not able to protect you from.
If you are going to invest all that money in SSDs, why would you cheap out on the system board and CPU so that you are not even able to use ECC memory? That doesn't add up in my mind.
There is a British saying I always loved, "penny wise, pound foolish." which I understand to mean, "making decisions over small amounts of money (pennies) that end up making bad sense with larger amounts of money (pounds, as in the money they used in Great Brittan before the Euro).
That brings me to the comment that
@rvassar made about your I/O.
You need a system board that has more PCIe lanes and has them allocated to slots in a manner that is more logical for the use of a server.
The following is meant to be an example, especially as some of these auctions are over and the items are not available. I just want to point you at the kind of hardware you should be looking at.
SuperMicro X9SRL-F Motherboard - listed as new - photos show a CPU cooler, but they don't say in the description
https://www.ebay.com/itm/202340882106
Price: US $279.99
SAMSUNG 16GB PC3L-12800R DDR3-1600 ECC Registered 1.35V RDIMM...
https://www.ebay.com/itm/302110582298
Price: US $54.95 x 8 = $439.60 for 128GB of RAM !!
Dynatron R27 Side Fan CPU Cooler 3U for Intel Socket LGA2011 (Narrow ILM)
https://www.ebay.com/itm/401284811045
Price: US $39.59
Intel Xeon E5-2650 V2 2.6GHz 8 Core 20MB 8GT/s SR1A8 LGA2011 (CM8063501375101) Processor
PassMark score of 13073... If you are wondering...
https://www.ebay.com/itm/283019094038
Price: US $96.95
For the drive controller, I would suggest a SAS controller, there are 4 SCA ports on the system board in addition to the SATA ports, but only two of the SATA ports are SATA III, so you will still be a little short.
SAS PCI-E 3.0 HBA LSI 9207-8i P20 IT Mode for ZFS FreeNAS unRAID
https://www.ebay.com/itm/162862201664
Price: US $69.55
Also, you will need forward breakout cables from the controller to the drives. I like these because they are a bit more durable than some of the slim ones that I have accidentally broken before:
Lot of 2 Mini SAS to 4-SATA SFF-8087 Multi-Lane Forward Breakout Internal Cable
https://www.ebay.com/itm/371681252206
Price: US $12.99
You might need to read this:
Don't be afraid to be SAS-sy
https://forums.freenas.org/index.php?resources/don't-be-afraid-to-be-sas-sy.48/
I would suggest one of these for the boot drive. It will last as long as the server, if not longer:
Intel-SSD-DC-S3500-Series-2-5-SATA-6Gb-s-20nm-MLC-80GB-SSDSC2BB080G4
https://www.ebay.com/itm/283017365544
Price: US $29.99
If you are using this for iSCSI, you are going to need a SLOG device and there are a number of discussions about that on the forum.
Here is one:
https://forums.freenas.org/index.php?threads/slog-benchmarking-and-finding-the-best-slog.63521